1. Field of the Invention
The present invention relates to an output buffer to drive external circuits connected to the output buffer through output pads of a semiconductor integrated circuit, to a semiconductor integrated circuit incorporating the output buffer and to a driving ability adjustment method for control the driving ability of the output buffer.
2. Description of the Related Art
In the prior technology relating to output buffers, there is a conventional output buffer as shown in FIG.1, for example.
FIG. 1 is a diagram showing a configuration of a semiconductor integrated circuit having a conventional output buffer. A signal S11 output from an internal circuit 101 in the semiconductor integrated circuit 100 is provided to an external circuit (not shown) placed out of a package enclosing this semiconductor integrated circuit 100 through an output buffer 102 and an output pad 103.
The conventional output buffer 102 comprises an inverter 110 for inverting the voltage level of the signal S11 transferred from the internal circuit 101. In the conventional output buffer 102, a plurality of inverters such as the inverters 111, 112 and 113 are incorporated between a node N11 as the output side of the inverter 110 and a node N12 connected to the signal output pad 103 through which the output signal S12 is transferred to the external circuit (not shown).
Next, the operation of driving an external circuit (not shown) connected to the conventional output buffer 102 having the above configuration through the signal output pad 103 will be explained.
FIGS. 2A and 2B are diagrams showing signal wave differences between conventional output buffer whose driving abilities are different to each other. FIG. 2A shows a signal wave when the output buffer drives an external circuit including a load capacity Ca. FIG. 2B shows a signal wave when the output buffer having the same driving ability drives an external circuit including a load capacity Cb. In those cases, there is the relationship that Ca&lt;Cb. In both FIGS. 2A and 2B, the reference character Vth designates a threshold voltage level of an input buffer of an external circuit.
As clearly shown in both FIGS. 2A and 2B, the rising time of the signal is different between the two cases because the load capacity is different to each other between those two cases. Accordingly, in order to obtain the most suitable rising time "t", an output buffer having the most required driving ability must be used for the external circuit.
However, in general, it is difficult to know that an output buffer in a semiconductor integrated circuit will be connected to an external circuit having a load capacity when the semiconductor integrated circuit having the output buffer is manufactured. In order to eliminate this problem, during its fabrication process in the prior art, it is assumed that an external circuit having an load capacity will be connected to a semiconductor integrated circuit having an output buffer which can drive this external circuit adequately.
However, there is a following drawback in the conventional output buffer incorporated in the semiconductor integrated circuit.
That is, in the conventional semiconductor integrated circuit, because the driving ability of the conventional output buffer is determined by assuming that a load capacity of an external circuit, connected to the signal output pad 103 in the semiconductor integrated circuit 100, to be driven by the output buffer is a suitable value, there is a case that an external circuit having a load capacity which is smaller than that of the assumed load capacity will be connected to the output buffer. Because the power consumption of the output buffer 102 is increased according to the magnitude of this driving ability, it causes the waste of the power consumption of the semiconductor integrated circuit when the driving ability of the output buffer is unmatched to the load capacity of the external circuit driven by the output buffer.